Carbon fiber is used in a variety of composite materials, by virtue of its excellent properties such as high strength, high elastic modulus, and high electrical conductivity. In recent years, in conjunction with developments in electronic techniques, carbon fiber has been considered a promising electrically conductive filler for producing electromagnetic wave shielding materials or antistatic materials, and has been viewed as a useful antistatic filler which can be incorporated into resin or as a promising filler employed in transparent electrically conductive resin. Also, by virtue of its excellent tribological characteristics and high wear resistance, carbon fiber has been considered as a promising material which is applicable for use in, for example, electric brushes and variable resistors. In addition, carbon fiber has become of interest as a wiring material for producing devices such as LSIs, since it exhibits high electrical conductivity, high thermal conductivity, and electromigration resistance.
Conventional carbon fiber produced through carbonization of organic fiber by means of heat treatment in an inert atmosphere, such as polyacrylonitrile (PAN)-based carbon fiber, pitch-based carbon fiber, or cellulose carbon fiber, has a relatively large diameter; i.e., 5 to 10 μm, and exhibits poor electrical conductivity. Therefore, such carbon fiber has been widely employed as a reinforcement material in, for example, resin or ceramic.
In the 1980's, studies were conducted on a process for producing vapor grown carbon fiber through thermal decomposition of a gas of, for example, hydrocarbon in the presence of a transition metal catalyst. Through such a process, vapor grown carbon fiber having a diameter of about 0.1 to about 0.2 μm (about 100 to about 200 nm) and an aspect ratio of about 10 to about 500 has been produced. A variety of processes for producing vapor grown carbon fiber are disclosed, including a process in which an organic compound such as benzene, serving as a raw material, and an organo-transition metallic compound such as ferrocene, serving as a catalyst, are introduced into a high-temperature reaction furnace together with a carrier gas, to thereby produce vapor grown carbon fiber on a substrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700); a process in which vapor grown carbon fiber is produced in a dispersed state (Japanese Patent Application Laid-Open (kokai) No. 60-54998, U.S. Pat. No. 4,572,813); and a process in which vapor grown carbon fiber is grown on a reaction furnace wall (Japanese Patent No. 2778434).
Since vapor grown carbon fiber is formed of carbon which is readily graphitized, when the carbon fiber is subjected to heat treatment at 2,000° C. or higher, the resultant carbon fiber exhibits excellent crystallinity and improved electrical conductivity. Therefore, the thus-graphitized carbon fiber is employed as an electrically conductive filler material in, for example, a resin or an electrode of a secondary battery.
A characteristic feature of each fiber filament of vapor grown carbon fiber resides in its shape and crystal structure. The fiber filament has a cylindrical structure including a very thin hollow space in its center portion, and a plurality of carbon hexagonal network layers grown around the hollow space so as to form annual-ring-like tubes. When vapor grown carbon fiber is subjected to heat treatment at 2,000° C. or higher, the cross section of each fiber filament of the thus-treated carbon fiber assumes a polygonal shape, and in some cases, micropores are formed in the interior of the fiber filament.
Since vapor grown carbon fiber has a small diameter, the carbon fiber has a relatively high aspect ratio. Generally, fiber filaments of the carbon fiber are entangled with one another to form fuzzball-like agglomerates.
Since vapor grown carbon fiber contains thermally decomposed carbon layers, the carbon fiber has a smooth surface. When such vapor grown carbon fiber is thermally treated at 2,000° C. or higher in an inert atmosphere, the thus-treated carbon fiber exhibits high crystallinity, and smoothness of its surface is further enhanced. The carbon fiber which has undergone heat treatment at high temperature has virtually no functional groups on its surface.
Since fiber filaments of vapor grown carbon fiber are entangled with one another to form agglomerates like fuzzballs, when the carbon fiber is mixed with a matrix formed of, for example, resin or ceramic, the carbon fiber fails to be uniformly dispersed in the matrix, and thus electrical, thermal, and mechanical characteristics of interest cannot be obtained.
When such carbon fiber having a high aspect ratio is mixed with a resin so as to form a composite material, and the surface of the composite material is observed under a scanning electron microscope, the surface of the composite material is found to be not smooth but “hairy” with pieces of the carbon fiber not covered with resin. When the composite material is employed as an antistatic material for producing, for example, an integrated circuit (IC) tray, due to generation of microscratches at a point at which the tray is in contact with a disk or wafer, or deposition of impurities caused by falling of the carbon fiber, the quality of the disk or wafer is lowered, and the yield of a final product is reduced.
When carbon fiber exhibits insufficient wettability and affinity to a matrix formed of, for example, resin, adhesion between the carbon fiber and the matrix is lowered. Therefore, mechanical strength of the resultant composite material is lowered, falling of the carbon fiber occurs, and the quality of the composite material is deteriorated.
In view of the above problems, various attempts have been made to reduce the length of long carbon fiber through grinding, in order to improve dispersibility of the carbon fiber and to obtain a composite material of smooth surface in relation to the use as a filler. Conventionally, carbon fiber has been ground through dry grinding by use of, for example, a ball mill, to thereby form short carbon fiber (Japanese Patent Application Laid-Open (kokai) No. 1-65144, U.S. Pat. No. 4,923,637 and Japanese Patent Application Laid-Open (kokai) No.11-322314). However, grinding of carbon fiber through impact grinding by use of, for example, a ball mill or a roll mill involves the following problems. Although entangled fiber filaments of the carbon fiber are fragmented through such grinding, fine carbon fiber fragments generated through grinding form agglomerates in a mill or the fragments are bonded together when grinding reaches a certain degree. Therefore, micronization of the carbon fiber does not proceed further, even if grinding is performed for a long period of time. In addition, the resultant carbon fiber fragments have a length as large as about some μm.